1. Field of the Invention
The present invention relates to a high pressure discharge lamp with a reflector that is suitable for use as a light source of a projector or the like.
2. Description of the Related Art
To achieve high luminance, recent discharge lamps raise the vapor pressure in the arc tube to 200 atmospheres or higher when they are lighted. Thus, if the temperature in the arc tube is raised by some cause, the pressure in the arc tube is also raised, and the arc tube becomes likely to burst. Such a high pressure discharge lamp typically has a concave reflecting mirror referred to commonly as reflector (referred to as reflector hereinafter). The reflector is typically made of borosilicate glass.
In the case where such a high pressure discharge lamp with a reflector is used in an apparatus, such as a liquid crystal projector and a projection television set, if the high pressure discharge lamps burst, there is a possibility that broken pieces of the arc tube hit the reflector to cause a crack or chip therein, or broken pieces of the reflecting mirror, in turn, damage other components of the apparatus.
Thus, there has been known a light source for a projection apparatus that has a reflector having an outer surface covered with a heat-resistant organic coating for preventing occurrence of a crack or chip in the reflector even if the lamp bursts (see the patent literature 1).
In addition, there has been known a method of using crystallized glass, which has high heat resistance and mechanical strength, as a shatter-proof reflector material, thereby minimizing the damage of any burst of the high pressure discharge lamp (see the patent literature 2).
In addition, there has been known a method of preventing fracture of the reflector in the case where the high pressure discharge lamp bursts by arranging a metal material between reflector base materials (see the patent literature 3).
[Patent literature 1] Japanese Patent Laid-Open No. 2000-47327
[Patent literature 2] Japanese Patent Publication No. 7-92527
[Patent literature 3] Japanese Patent Laid-Open No. 2004-354425
Even if a heat-resistant organic coating is formed as described in the patent literature 1, if the heat-resistant organic coating is made of a fluorine-based resin, it can resist only about 260 degrees Celsius. Thus, the rated power of the lamp is limited to about 200 W at the most. On the other hand, if the heat-resistant organic coating is a polyimide-based coating, the heat resistance is improved to about 400 degrees Celsius. However, the polyimide-based resin is expensive, so that there is a possibility that the manufacture cost relatively increases.
It is said that, if crystallized glass is used as described in the patent literature 2, the brightness is compromised because the crystallized glass is inferior to borosilicate glass in inner surface precision. In addition, the crystallized glass is more expensive than the inexpensive borosilicate glass, so that there is a possibility that the manufacture cost relatively increases.
If a metal material is used as described in the patent literature 3, infrared light once transmitted to the rear of the reflector base material is emitted toward the front again to damage not only resin parts of the optical unit but also optical parts (a lens, a fly's eye lens, a light pipe or the like).